In digital pathology and particular in the case of whole slide scanning, specimens are sliced and imaged for analysis purposes as well as teaching purposes. Line sensors may be used for scanning a whole tissue slide. These slide scanners may perform a continuous mechanical scanning, thereby reducing stitching problems and allowing for the use of so-called time delay integration (TDI) line sensors in order to accommodate for low brightness of the illumination.
For focusing focus maps may be used. Before the actual scanning the optimum focus position is determined at a number of positions on the slide. This results in a “focus map”. This procedure may be necessary because the axial position of the tissue layer may vary with several micrometers across the slide, as may be seen in FIG. 1. The variation of the tissue layer may thus be more than the focal depth of the microscope objective. During scanning the focus position of the objection is set on a trajectory that interpolates between the measured optimum focus settings on the selected measurement locations. This procedure may be both prone to errors and be also time-consuming, thereby limiting the throughput of the system.
WO 2005/010495 A2 describes a system and a method for generating digital images of a microscope slide, the microscope comprising a main camera and a focus camera which is tilted with respect to the optical axis.